The present invention relates generally to apparatus used to dispense a plurality of liquids through a single orifice, and more particularly, to the dispensing gun and removable mixing chamber used to dispense the chemical reactants required to form polyurethane foams.
Polyurethane foams are formed by the reaction of an isocyanate component and hydroxyl-bearing compounds. When mixed in the presence of a catalyst and other additives, such as a polyether resin, a surfactant, a catalyst, and a blowing agent, these chemicals react to form cross-linked polymer chains, more commonly known as a polyurethane. Each of these components of the plural component material, by itself, is generally stable. Thus, each component will not cure or cross-link for extended periods of time, often as long as several months, if they are properly stored. However, when the isocyanate component and the chemical polyol component, a preformulated compound formed from the aforementioned resin, surfactant, catalyst and blowing agent, are mixed together in proper concentrations, an exothermic chemical reaction of the isocyanate and polyol occurs. This reaction causes a continued expansion that is evidence of the polymerization and manifests itself as foam which cross-links and cures. The cross-linking and curing usually is substantially completed in a matter of seconds.
Polyurethane foam dispensers are well known and have achieved a high degree of usage in factories where components must be adhesively lined with an insulating foam or where products must be packaged and protected from damage during shipment. This high level of use of polyurethane foam dispensing equipment has also focused attention on the efficiency of prior apparatus employed to accomplish this dispensing. Many of the problems with foam dispensing equipment stem from the fact that the polyurethane foam "sets" or builds up in the dispenser, normally within the mixing chamber or the nozzle, after the chemical components have cross-linked and begun to cure. This "setting" can eventually cause the apparatus to become inoperative due to clogging or blockage of the flow passages.
In operations such as those required during packaging, where intermittent use of the dispensing apparatus is required, the "setting" problem is more severe. This typically occurs in the situation where a packer initially directs a "shot" of the mixed plural components into the bottom of a container, inserts a polyethylene strip over the top, and places the product to be shipped in the container. Another sheet of polyethylene is placed on top of the product, and the foam dispensing apparatus is then activated after a delay of 10 to 20 seconds from the time the first "shot" was dispensed to fill the box with the cushioning foam. This procedure is repeated for each item to be packed.
Prior foam dispensing apparatus has attempted to solve this "setting" problem by using either separately or combinatively air blasts, cleaning rods or plungers with scrapers, or solvent to remove the residue foam from the dispensing assembly. Specific polyurethane foam systems have attempted to use air with pressurized solvent blown into the mixing chamber and the dispensing nozzle, an automatic solvent flush that runs through the mixing chamber and dispensing nozzle, a blast of purging gas preceding the continuous pumping of solvent through the mixing chamber and dispensing nozzle, and reciprocating cleaning rods or plungers which may or may not use the solvent to facilitate the scraping or residue foam from the dispensing apparatus.
The use of a combination cleaning rod and valving rod to control the flow of the polyol and isocyanate components has proven to be the most common design employed in commercial applications. Since most of the clogging from foam buildup occurs within the mixing chamber or nozzle, designs have evolved employing removable mixing chambers and dispensing nozzle members.
Some designs have employed levers which permit the nozzle to be removed from the dispensing gun. Once these components are removed from the dispensing guns, they are frequently soaked in solvent to reduce or remove the residue foam. The solvent, however can attack gaskets or O-ring seals in the components and ruin the tight seals necessary.
Other designs have employed a TEFLON.RTM. core in the mixing nozzle to overcome cold forming problems that typically result when the core material gradually changes shape under loading to reduce the stress. Cold forming problems affect the tight sealing necessary in interference fittings with the valving rods, thereby permitting the leakage of the isocyanate and polyol components The mixing nozzle is restrained at each end of the core in both an axial and a radial direction. Another design has employed a detachable mixing chamber and a valving rod as one integral unit.
All of these units with removable mixing chambers suffer from the deficiency of having mixing chambers that are large and costly, or which require the disposal of both the mixing chamber and the valving rod should failure of one component occur. The use of metal parts in the mixing chamber to axially and radially restrain the TEFLON.RTM. core at each end, as has been done in one design, also has increased the cost of the foam dispensing apparatus and, specifically, the removable mixing chambers.
These problems are solved in the design of the present invention by providing in a plural component dispensing assembly a removable mixing chamber and a valving rod that are separable and which both may be formed of a low friction plastic which is easily discardable.